1. Field of Invention
The present invention relates to circuit boards for electronic components and more particularly to a circuit board having encapsulated wires which are bonded to a substrate by a photo-curable adhesive.
2. Description of Prior Art
The art of making circuit boards is well developed. The technique most widely used to fabricate circuit boards is generally referred to as the subtractive processes. This process begins with a sheet of copper on a substrate. The copper sheet is etched to form a pattern of desired circuit paths. A more advanced technique for fabricating circuit boards is termed the additive process. The additive process begins with an insulating substrate and adds copper where needed to form the desired circuit lines. One way of practicing the additive process to fabricate circuit boards is described in U.S. Pat. No. 4,030,190.
An alternative way to fabricate circuit boards involves encapsulating insulated wires in an adhesive. The resulting circuit board is generally termed an encapsulated wire circuit board. Encapsulated wire circuit boards are commercially available under the trade name "Multiwire" which is owned by the Kollmorgen Corporation. The encapsulated wire technique is shown in U.S. Pat. Nos. 4,097,684 (Burr); 3,646,772 (Burr); 3,674 914 (Burr) and 3,674,602 (Keough).
Co-pending application Ser. No. 392,996 by Varker entitled "High Density Encapsulated Wire Circuit Board" describes a technique for making an encapsulated wire circuit board wherein the insulated wires are firmly bonded to a relatively thick layer of copper which is covered by a layer of pre-preg. The expansion and contraction of the board during thermal cycling is controlled by the copper and therefore there is not any significant amount of unpredictable variations in the dimensions of the board. Holes can be very accurately drilled at precise locations in such boards. In the technique shown in the co-pending Varker application insulated wires are bonded to a substrate utilizing a heat curable adhesive so that after the adhesive is cured the wires can not move relative to the substrate.
The aforementioned patent application Ser. No. 392,998, now U.S. Pat. No. 4,427,478, "Process for Making an Encapsulated Circuit Board and Products Made Thereby" by Grant et al describes an improvement of the circuit board and process described in the Varker co-pending application. The Grant et al application describes the fabrication of an encapsulated wire board using a photo-curable adhesive material. In the technique described in the Grant et al patent application, the wires are laid in a photo-curable adhesive, this adhesive is exposed to light, thereby curing the adhesive and firmly bonding the wires to the substrate. The Grant et. al. application describes the use of the material describes in U.S. Pat. No. 4,169,732 (Shipley). The material described in U.S. Pat. No. 4,169,732 does not have the unique combination of rheological properties of the photo-curable material required to practice the present invention.